feat(block-editor): add Test Block preview

Preview the current unsaved block in a disposable drawing with independent history. Keep normal Draw tools available, append Close Test Block, and return to the original editor without saving. Guard persistence paths and handle refresh and parent-close lifecycle by stable document identity.

src/app/commands/fileops.rs

@@ -38,7 +38,7 @@ impl OpenCADStudio {
                     let mut saved = 0usize;
                     let mut skipped = 0usize;
                     for t in 0..self.tabs.len() {
-                        if self.tabs[t].is_start {
+                        if self.tabs[t].is_start || self.tabs[t].block_test.is_some() {
                             continue;
                         }
                         if let Some(path) = self.tabs[t].current_path.clone() {

src/app/commands/inquiry.rs

@@ -614,6 +614,8 @@ impl OpenCADStudio {
             "BSAVE" => {
                 self.save_block_edit(i);
             }
+            "BTESTBLOCK" => return Some(self.open_block_test(i)),
+            "BCLOSETESTBLOCK" => return Some(self.close_block_test(i)),
             "BSAVEAS" => {
                 self.open_block_save_as_dialog(i);
             }

src/app/commands/mod.rs

@@ -149,6 +149,33 @@ impl OpenCADStudio {
         };
         let resolved = self.resolve_alias(cmd);
         let cmd = resolved.as_deref().unwrap_or(cmd);
+        if self.tabs[i].block_test.is_some() {
+            let name = cmd.split([' ', ':']).next().unwrap_or(cmd);
+            if matches!(
+                name,
+                "BEDIT"
+                    | "BEDIT_BEGIN"
+                    | "BEDIT_OPEN"
+                    | "REFEDIT"
+                    | "REFEDIT_BEGIN"
+                    | "BSAVE"
+                    | "BSAVEAS"
+                    | "SAVE"
+                    | "QSAVE"
+                    | "SAVEAS"
+                    | "SAVEALL"
+                    | "WBLOCK"
+                    | "EXPORT"
+                    | "ETRANSMIT"
+                    | "ARCHIVE"
+            ) {
+                self.command_line.push_error(
+                    crate::t!("Close Test Block before saving or opening another block editor.")
+                        .as_ref(),
+                );
+                return Task::none();
+            }
+        }
         if is_spacemouse_command(cmd) {
             return self.run_action(cmd);
         }